The trellis coding scheme defined for discrete multitone (DMT) asymmetric digital subscriber line (ADSL) systems (ITU Standard G.992.1) requires that data modulated tones carry at least two bits. Although fine transmit gain adjustment can be used to compensate for integer truncation effects in bit loading, the allowable fine gain adjustment range is insufficient to compensate for bit loading truncation from 2-bits to 0-bits. As a result, trellis coded DMT systems are unable to take advantage of the capacity available in tones that have insufficient signal-to-noise ratio (SNR) to support 2-bits, after allowing for fine gain adjustment.
Parallel transmission of information on several DMT tones has been proposed as a means of utilizing the capacity in low SNR sub-carriers. (xe2x80x9cParallel Transmission to Increase Realizable Data Rate in DMT-based Systems,xe2x80x9d NG-039, Nuremberg, Germany, August 1999.) Parallel transmission of information would provide redundant capability to some degree because fractional bit loading of tones can be effectively realized through fine transmit gain adjustment. If one-bit constellations are combined with fine gain adjustment, sub-carriers that support at least xc2xd bit at nominal transmit power are available for data modulation.
The present invention is directed to a method for combining trellis-coded modulation with one-bit constellations to make more efficient use of available channel capacity in discrete multitone (DMT) asymmetric digital subscriber line (ADSL) systems. Through fine transmit gain adjustment, in individual tones, DMT systems are able to compensate for truncation effects in bits-per-tone loading. However, the allowed fine gain adjustment range is insufficient to compensate for the DMT requirement that data modulated tones carry at least two bits. Therefore, by allowing data-modulated tones to carry a single bit, the available channel capacity can be more efficiently utilized.
Additionally, one-bit constellations may be combined with trellis coded modulation such that the primary design of the G.992.1 trellis encoder and decoder remain unchanged. A pair of one-bit modulated sub-carriers may be treated as a virtual two-bit modulated sub-carrier in the trellis encoder and decoder. At the transmitter, a trellis encoded bit pair is transmitted on two sub-carriers, using one-bit constellations, rather than a single sub-carrier using two-bit constellations. At the receiver, two one-bit sub-carriers are remodulated to form a single two-bit sub-carrier prior to trellis decoding. In this manner, the G.922.1 trellis encoder and decoder (issue 4.6.2), which typically require two-bit constellations for proper operation, may be made to operate properly with one-bit constellations.
Simulation results show imperceptible differences, in ADSL upstream and downstream data capacities, for pure fractional bit loading versus xc2xd bit-per-tone minimum loading. In addition, a simple technique is shown for extending trellis coded modulation to single-bit constellations.